On Sun, Feb 10, 2013 at 10:15:14AM -0800, Shentino wrote:
> Quick question from a n00b.
>
> What are "credits"?
>
> I imagine I'm not the only onlooker curious about ti either.
A group of operations which need to be applied to the file system as
an atomic set of block updates are grouped by handles. This is very
to "BEGIN TRANSACTION" and "END TRANSACTION" pairs in SQL. For example:
BEGIN TRANSACTION
INSERT INTO INODE_TABLE (ino, i_blocks, i_size, i_uid) VALUES
(11, 2, 1024, 12);
UPDATE INODE_ALLOCATION_BITMAP SET in_use=TRUE where ino=11;
END TRANSACTION
The reason why we use the terminology "starting a handle" instead of
"starting a transaction" is because a large number of handles are
bundled together to form a jbd2 transaction. This is because commits
are expensive, and for performance reasons we batch a large number of
handles, or perhaps what you might call "micro-transactions" into a
one large jbd2 transaction, which in general gets commited once every
five seconds or so, unless a commit is explicitly requested, or we
start running out of space in the journal.
In order to make sure that we don't run out of space, before we start
a handle, we have to declare how the maximum number of blocks we are
planning on modifying --- this reservation of space in the journal is
also referred as journal credits, since each time we call
ext4_journal_get_write_access(), if that block is not already part of
the transaction, the number of credits associated with the handle will
be decremented by one.
When we open a handle, if there is not sufficient space (credits) left
in the current transaction, then start_this_handle() will request that
the current transaction be committed, and then block until the a new
transaction can be started.
When we are done, we close the handle. If it turns out that we didn't
need to modify as many blocks as we had declared when we opened the
handle, or if some of the blocks that we modified were already part of
the transaction, those excess credits are returned to the transaction.
So as an example, suppose we need to modify two metadata blocks
associated with an inode. That sequence might look like this:
/* start a handle with two credits */
/* note: this can trigger a journal commit if there
is not enough space left in the journal */
handle = ext4_journal_start(inode, 2);
ext4_journal_get_write_access(handle, bh);
<modify the metadata block found in bh>
ext4_handle_dirty_metadata(handle, inode, bh)
ext4_journal_get_write_access(handle, bh2);
<modify the metadata block found in bh2>
ext4_handle_dirty_metadata(handle, inode, bh2)
/* note: this can trigger a journal commit if the handle
is marked synchronous, or if enough time has elapsed --- i.e.,
the 5 second commit timer. */
ext4_journal_stop(handle);
In the case where we are running in no journal mode,
ext4_journal_start(), ext4_journal_get_write_access(), and
ext4_journal_stop() are effectively no-ops, and
ext4_handle_dirty_metadata() is mapped to mark_buffer_dirty_inode(bh,
inode), or mark_buffer_dirty(bh) if inode is NULL. (inode is NULL if
we are modifying metadata blocks belonging to a specific inode, such
as a block group descriptor block or an inode table block.)
Hope this helps / is interesting to those who are following along at
home. :-)
- Ted
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